Male Bluefin killifish communicate with other members of the species using their fins, a new study has found.

Despite its common name, the fin colours of male Lucania goodei vary, with red, yellow and/or black markings on their anal, caudal (tail) and dorsal fins.

University of Illinois animal biology professor Rebecca Fuller noticed this when snorkelling in a stream in Florida.

"In some of the males, the anal fin was yellow, and then some of them were red," she said. "And the field guide showed them as blue."

Some of the males had darker black markings on their anal fins than others, and some had bright yellow or orange caudal fins.

Fuller decided to try and find out what caused this variation in colour.

Previous studies suggested that melanin, the black pigment, is a badge of status among males; those with more prominent melanin markings tending to be more aggressive towards other males. In the new study, Fuller and her former graduate student Ashley Johnson, found that males with heavier melanin outlines on their anal fins dominated, driving other males away to gain exclusive access to females.

"Melanin is a signal to other males: 'I’ve been winning in the past and I'm doing well and get out of my way,'" Fuller says.

The red and yellow pigments on the anal fins and the yellow tints on the caudal fins have different origins, the researchers found. Carotenoids colour the caudal fins, but pterins tint the anal fins either yellow or red.

Carotenoids are known antioxidants; they gobble up highly reactive ions or molecules that can damage cells and tissues. Because killifish obtain carotenoids only by eating, researchers hypothesise that a display of colour derived from carotenoids signals to potential mates that the male in question is robust and well-fed.

In the new study, Fuller and Johnson discovered that richer carotenoid colouration on the caudal fin was associated with better body condition, lower parasite infection and good spawning success, suggesting that females respond positively to the brightly pigmented tail fins of potential mates.

Much less is known about pterins, Fuller says. They are associated with immune function and also have antioxidant characteristics, and so also may be a badge of health.

In the new study, the researchers found that carotenoids coloured only the caudal fin, while pterins appeared only on the anal fin. Brighter pterin colouration was associated with lower parasite infection and higher spawning success.

"We are finding that communication is complicated in nature and that animals have evolved ways to send different messages to different receivers," Fuller explains. "In the case of Bluefin killifish, multiple messages are being provided by three distinct pigments that are in three different areas of the body. Both females and males are getting these messages. Males are paying attention to the melanin, most likely, and females are paying attention to these more-colourful fins."

The results of the study are published in the journal Behavioral Ecology.